Final answer:
Scientists recommend the removal of aging dams to clear blockages in rivers, a practice that has led to increased fish diversity and returning populations of species like salmon and American shad.
Step-by-step explanation:
One major recommendation of scientists to remove the blockages to fish migration caused by dams and help restore the American shad is the removal of aging dams. Since the mid-1980s, shifting beliefs about the ecological value of free-flowing rivers have led to the consideration of dam removal as opposed to replacement. The benefits include restored natural water levels, leading to increased fish diversity and improved water quality. In the Pacific Northwest, expected outcomes also involve the increased populations of salmon, a keystone species. Similarly, on the Atlantic coast, the removal of dams has facilitated the return of other anadromous fish species, contributing to the restoration of natural fish assemblages.